1. Field of the Invention
This invention relates to an air powder classifier and more particularly to a classifier suited especially for a strongly adhesive ultrafine ceramic powder. The classifier balances a centrifugal force which is developed by the rotation of a turbine-like classifying rotor acting on particles of the powder and a counter-acting carrying force of an air flow which flows in the direction contrary to the centrifugal force.
2. Description of the Prior Art
Powder classifiers of the type using a turbine-like classifying rotor have been known as air powder classifiers suited for powders such as ceramic powders, etc. measuring several .mu.m to 100 .mu.m. A typical example of the known powder classifiers of this type is arranged as shown in FIGS. 1 and 2 of the accompanying drawings. As shown, the classifier has a unified combination of a classifying rotor 4 and a balance rotor 5 which is rotated around a vertical rotating shaft 3 and interposed in between an upper casing 1 and a lower casing 2. In the peripheral part of the classifying rotor 4, there is formed a radial passage type classifying chamber C. The chamber C in this case consists of many classifying blades 6 and 7 which are arranged in a radial configuration of inner and outer arrays serving as many radial partitions. The outer circumferential opening of the classifying chamber C opens to a circular passage 8 which is formed with the upper casing 1. Further, a circular clearance 9 is provided on the upper side of the classifying chamber C. In the middle part of the upper side of the upper casing 1 is provided a powder pouring inlet 10. A powder material supplied from this pouring inlet 10 is turned and moved toward the outer side of the diameter into the classifying chamber C by dispersing blades 11 which are arranged to turn and are radially formed. The upper surface of the classifying rotor 4 is opposed to the dispersing blades 11 and to the inner surface of the upper casing 1 in such a manner that the powder supplied is turned and primarily dispersed by means of the dispersing blades 11. The primarily dispersed powder is then secondarily dispersed into an evenly dispersed state by a clearance part 12 between the inner surface of the upper casing 1 and the upper surface of the classifying rotor 4.
Further, the passage type classifying chamber C of the classifying rotor 4 bends downward on the inner side of the diameter and communicates with a spiral casing part 14 through a guide passage 13 formed on the balance rotor 5 and also through the peripheral part of the balance rotor 5. The spiral casing part 14 is connected to a collector such as a cyclone, a bag filter, etc. and is arranged to have negative pressure of an air flow produced by a suctorial action of the balance rotor 5 or by a blower or the like which is not shown.
The classifier further includes an air intake port 15 which is arranged between the upper and lower casings 1 and 2 to guide air into the above stated circular passage 8. The flow of air introduced via the intake port 15 is arranged to flow only toward the inside of the circular passage 8 by virtue of the lower end flange 16 of the upper casing 1 and radial auxiliary blades 17 of the classifying rotor 4. A rotating shaft bearing 18 is arranged to carry a rotating shaft 3. A powder flow deflecting member 19 is arranged below the powder pouring inlet 10 provided in the upper side of the classifying rotor 4.
When the powder material is continuously poured into the classifier via the powder pouring inlet 10 with the rotating shaft 3 allowed to rotate at a predetermined speed, the poured powder is subjected to the primary dispersion effected by the dispersing blades 11. The primarily dispersed powder is then subjected to the secondary dispersion and enters the classifying chamber C via the circular clearance 9. Within the chamber C, a centrifugal force which is produced by the turning classifying blades 6 and 7 and an air flow which takes place due to suction exerted from the guide passage 13 to the inside relative to the diameter of the rotor 4 act on the powder. The centrifugal force causes a corse powder portion of the powder material having a larger particle size to flow outward to the circular passage 8 disposed outside the diameter of the rotor 4. Meanwhile, since the centrifugal force acts to a less degree on a fine powder portion of the powder material having a smaller particle size, the air flow carries the fine powder into the spiral casing part 14 via the passage 13. The power material thus can be continuously classified. The fine powder thus classified and separated by the classifier is supplied from the spiral casing part 14 to a collector which is a bag filter or the like. Meanwhile, the coarse powder caused to flow out to the circular passage 8 by the centrifugal force is supplied to a coarse powder recovering device via the discharge port 20 (FIG. 2) which is formed in the outer circumferential wall of the circular passage 8 and also via a duct line which extends from the discharge port 20 outward or in the tangential direction relative to the outer circumferential wall.
However, with the conventional powder classifier arranged in this manner, there have been frequent occurrences of troubles caused by powder sticking to the wall surface of the classifier. The powder sticking troubles have frequently occurred especially in the coarse powder recovering system. To solve this problem, the present inventors contrived a powder classifier wherein the duct line which guides the coarse powder from the circular passage to the coarse powder recovering device is arranged to branch out halfway and to have the branch extend back to the circular passage. The reason for the branching out of the duct line and the bringing of the branch back to the circular passage is that the coarse powder recovering device is generally operated with a rotary valve or the like in an air sealed state. This results in a stagnant air flow and positive pressure within the duct line. Under such a condition, the powder tends to stick to the inner wall surface of the duct line. Therefore, the above stated connection arrangement to bring back a portion of the duct line of the coarse powder recovering system prevents the air flow from stagnating and from aiding the powder in sticking to the duct wall halfway in the duct line, so that the above stated inconvenience can be moderated to a great extent. The present inventors conducted many tests on the powder classifier arranged in the above stated manner. As a result of the tests and studies, they found another important advantage of the classifier in addition to the prevention of the duct line from being clogged by the sticking of the powder. The advantage found resides in that the powder classifier tends to enhance a fine powder recovery percentage .eta.. The inventors have further conducted studies on the reason for the improvement in the fine powder recovery percentage. These studies have led to the present invention.